The expansible sheave and V-belt continuously variable transmission (or CVT) is a type of automatic gearbox for vehicles in which the transmission ratio can change without a break between two limit values. The CVT is widely used in small and medium powered two-wheel vehicles, in particular mopeds. The reasons for such extensive diffusion are to be sought in the advantages offered by this type of transmission in terms of:                economical production;        ease of construction and maintenance;        performance and reliability.        
Recent progress in the technology of materials has further permitted the development of new types of belts, based on the use of polymer supports reinforced with high resistance and flexible fibres and textiles, and characterised by high performance in terms of transmissible power and performance.
The use of such belts, together with a general increase in knowledge of the functioning of this type of transmission, related primarily to the progressive build-up of experience accumulated by the manufacturers, has permitted significant increases in performance to be achieved both in terms of operating reliability and of efficiency.
The continuous speed control performs its function of changing the transmission ratio by making the winding diameters of the belt on two pulleys vary, of which one is the drive force and the other is driven. The motion coming from the drive shaft is transmitted by said two pulleys, at least one of which is able to draw together or distance the two parts or half-sheaves of which it is composed. This way the belt finds itself working at a higher or lower point of the sheave, thereby varying the transmission ratio. The belt being non-extensible and the distance between the sheaves being invariable, the minimum ratio (long gear) is obtained when the belt is in a minimum diameter condition on the drive sheave and maximum diameter on the driven sheave, while the maximum ratio (high gear) is obtained, conversely, when the belt rides on the maximum diameter of the drive sheave and minimum diameter of the driven sheave. The method of varying the aperture of the sheaves, which thus influences the gear change, provides the gear change law/for which the CVT is designed.
This type of automatic transmission is by now quite common for all the main transmission manufacturers, both as a functional and constructional layout. The drive sheave usually comprises a speed regulator made with centrifugal masses (rollers) which have the purpose of performing the aperture of the respective half-sheaves. The driven group instead comprises a sheave formed of the respective half-sheaves connected to each other by means of a simple contrast spring (motorcycle type CVT); or by means of a spring and a corrector or torque servo gear (moped type CVT). The automatic clutch, of the centrifugal mass type, is positioned in the driven group.
As well as this centrifugal transmission solution, there is also an electromechanical system on the market composed again of two sheaves and a connection belt between them, but with the novelty of an electric motor being introduced, accompanied by sophisticated electronic operation. The movement of the mobile half-sheave on the primary shaft is actuated by the electric motor and not by the weight of the blocks housed therein, as instead happens with traditional transmissions. This way the ratio can be managed as desired, according to the “map” loaded in the control unit of the electric motor.
The characterisation of a vehicle's drive unit, as regards pollutant emissions, currently constitutes a fundamental aspect of its development. Increasingly strict anti-pollution laws imposing limits on emission values, in fact often constitute a crucial constraint when defining the parameters for the global design and development of a drive unit.
The purpose of the present invention is therefore to make a transmission system, in particular a transmission system for motorcycles of the continuously variable type, fitted with a device for regulating the gear change curve which is able to reduce both fuel consumption and the pollutant emissions of the motorcycle.
A further purpose of the invention is to make a transmission system with device for regulating the gear change curve which is able to increase the adaptability of the motorcycle to the rider's needs.
A further purpose of the invention is to make a transmission system with device for regulating the gear change curve which is particularly simple and economical to make.
Further characteristics of the invention are highlighted in the dependent claims which form an integral part of the present description.